REVIEW Open Access iPS cell technologies: significance and

Okano and Yamanaka Molecular Brain 2014, 7:22http://www.molecularbrain.com/content/7/1/22

REVIEW Open Access

iPS cell technologies: significance andapplications to CNS regeneration and diseaseHideyuki Okano1* and Shinya Yamanaka2,3*

Abstract

In 2006, we demonstrated that mature somatic cells can be reprogrammed to a pluripotent state by gene transfer,generating induced pluripotent stem (iPS) cells. Since that time, there has been an enormous increase in interestregarding the application of iPS cell technologies to medical science, in particular for regenerative medicine andhuman disease modeling. In this review article, we outline the current status of applications of iPS technology tocell therapies (particularly for spinal cord injury), as well as neurological disease-specific iPS cell research (particularlyfor Parkinson’s disease and Alzheimer’s disease). Finally, future directions of iPS cell research are discussed includinga) development of an accurate assay system for disease-associated phenotypes, b) demonstration of causativerelationships between genotypes and phenotypes by genome editing, c) application to sporadic and commondiseases, and d) application to preemptive medicine.

Keywords: Induced pluripotent stem cell, Cell transplantation, Spinal cord injury, Modeling human diseases,Parkinson’s disease, Alzheimer’s disease

The development of induced pluripotent stem(iPS) cell technologies and their significanceThe 2012 Nobel Prize in Physiology or Medicine wasawarded for “The discovery that mature cells can bereprogrammed to become pluripotent.” First, we wouldlike to consider the significance of this research. Thelives of mammals, including humans, begin with thefertilization of an egg by a sperm cell. In humans, a blasto-cyst composed of 70-100 cells forms by approximately 5.5days after fertilization. The blastocyst is composed of theinner cell mass, the cell population that has the ability todifferentiate into the various cells that constitute the body(pluripotency), and the trophoblast, the cells that developinto the placenta and extra-embryonic tissues and donot contribute cells to the body. In the embryonic stage,the pluripotent cells of the inner cell mass differentiateinto the three germ layers, endoderm, mesoderm, andectoderm, which will form specific organs and tissuescontaining somatic stem cells with limited differentiationpotencies. These somatic stem cells continue to divide and

* Correspondence: [email protected]; [email protected] of Physiology, Keio University School of Medicine,35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan2Center for iPS Cell Research & Application, Kyoto University,53 Kawaharacho, Shogoin, Sakyoku, Kyoto 606-8507, JapanFull list of author information is available at the end of the article

© 2014 Okano and Yamanaka; licensee BioMeCreative Commons Attribution License (http:/distribution, and reproduction in any mediumDomain Dedication waiver (http://creativecomarticle, unless otherwise stated.

differentiate, and, by adulthood, an individual composedof 60 trillion cells is produced. Somatic stem cells born inthe fetal period actively divide, and are involved in the for-mation and growth of various organs. However, even inthe adult, somatic stem cells persist in niches in everyorgan and tissue, and play an important role in maintain-ing organ and tissue homeostasis. When cells in the innercell mass are removed at the blastocyst stage and culturedin vitro, pluripotent embryonic stem (ES) cells are ob-tained. Thus, in the normal process of development, celldifferentiation of the three germ layers proceeds fromthe simple stages of the fertilized egg and blastocyst,and ultimately produces an individual consisting of acomplex cellular society.In 1893, August Weismann argued that only germ cells

(eggs and spermatozoa) maintain a “determinant,” whichwas described as heritable information essential to decideon the functions and features of all somatic cells in thebody [1]. In his germ plasm theory, the determinants arelost or irreversibly inactivated in differentiated somaticcells.It took more than 50 years for researchers to rewrite

this dogma. In 1962, Sir John Gurdon demonstrated theacquisition of pluripotency by reprogramming cells totheir initial stage using a novel research technique, i.e.,

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producing cloned individuals by transferring somatic cellnuclei into eggs [2]. However, for many years, that resultwas regarded as a special case limited to frogs alone.The production of Dolly the sheep by transferring thenucleus of a somatic cell (mammary gland epithelial cell)by Sir Ian Wilmut in the late 1990s [3] showed thatcloning could also be applied to mammals.These brilliant previous works led to our studies that

culminated in the induction of pluripotency in mousesomatic cells in 2006, using retroviral vectors to intro-duce four genes that encode transcription factors i.e.,Oct4, Sox2, Klf4, and c-Myc. We designated these cellsas iPS cells [4]. In 2007, we succeeded in generatinghuman iPS cells using genes encoding the same fourtranscription factors [1]. The results of this researchshowed that although the developmental process wasthought to be irreversible, by introducing key genes intodifferentiated adult cells the cells could be reset to astate in the extremely early stage of development inwhich they possessed pluripotency. That is, the resultsdemonstrated that the differentiation process was revers-ible. This startling discovery made it necessary to rewritethe embryology textbooks.Three major lines of research led us to the production

of iPS cells [5] (Figure 1). The first, as described above,was nuclear reprogramming initiated by Sir John Gurdonin his research of cloning frogs by nuclear transfer in 1962[2] and by Sir Ian Wilmut, who cloned a mammal for thefirst time in 1997 [3]. In addition, Takashi Tada showedthat mouse ES cells contain factors that induce repro-gramming in 2001 [6]. The second line of research wasfactor-mediated cell fate conversion, initiated by HaroldWeintraub, who showed that fibroblasts can be convertedinto the muscle lineage by transduction with the MyoD

1962Cloning in frogGurdon

1CW

1987MyoDWeintraub

1981Mouse ES cellsEvans, Martin

1988LIFSmith

III)

I)

II)

Figure 1 The history of investigations of cellular reprogramming that2006 [4] became possible due to three scientific lines of investigation: 1) nu3) ES cells. See the text for details (modified from Reference [5] with permi

gene, which encodes a muscle lineage-specific basic helix-loop-helix transcription factor in 1987 [7]. The third lineof research was the development of mouse ES cells, ini-tiated by Sir Martin Evans and Gail Martin in 1981[8,9]. Austin Smith established culture conditions formouse ES cells and identified many factors essential forpluripotency including leukemia inhibitory factor (LIF)in 1988 [10]. Later, he developed the method to inducethe ground state of mouse ES cell self-renewal usinginhibitors for mitogen-activated protein kinase andglycogen synthase kinase 3 [11], which supports the es-tablishment of fully reprogrammed mouse iPS cells.Furthermore, James Thomson generated human ES cells[12] and established their optimal culture conditions usingfibroblast growth factor-2 (FGF-2). Without these previ-ous studies, we could never have generated iPS cells. Inter-est rapidly escalated, and, in tandem with the birth of iPScell technology, pluripotency leapt into the mainstream oflife sciences research in the form of “reprogramming tech-nology” [13]. However, there remain many unansweredquestions regarding reprogramming technology. What arethe reprogramming factors in the egg cytoplasm that areactive in cloning technology? What do they have in com-mon with the factors required to establish iPS cells andwhat are the differences? What kind of epigenetic changesoccur in association with the reprogramming?Apart from basic research in embryology, broad inter-

est has been drawn to the following possible applicationsof iPS cell research: (1) regenerative medicine, includingelucidating disease pathologies and drug discovery re-search using iPS cell disease models, and (2) medicaltreatments (Figure 2). In this review, we describe thesepotential applications in the context of the results of ourown research.

997loning in Sheepilmut

1998human ES cellsThomson

2006iPS cellsYamanaka

2001ES cells fusionTada

led to the development of iPS cells. Our generation of iPS cells inclear reprogramming, 2) factor-mediated cell fate conversion, andssion).

Cell therapy Disease modelingDrug development

Spinal cord injury

Parkinson disease

Alzheimer disease

ALS

iPS cell

Parkinson disease

Figure 2 The application of iPS cell technologies to medicalscience. iPS cell technologies can be used for medical scienceincluding 1) cell therapies and 2) disease modeling or drugdevelopment. See the text for details.


Applications of iPS cell technologies toregenerative medicineGeneral statement of iPS-based cell therapyiPS cells can be prepared from patients themselves andtherefore great expectations have been placed on iPS celltechnology because regenerative medicine can be imple-mented in the form of autografts presumably withoutany graft rejection reactions. Although there have beensome controversies [14], the immunogenicity of termin-ally differentiated cells derived from iPS cells can benegligible [15-17]. Moreover, there has been substantialinterest in the possibility of regenerative medicine with-out using the patient’s own cells; that is, using iPS cellstocks that have been established from donor somaticcells that are homozygous at the three major humanleukocyte antigen (HLA) gene loci and match the

Table 1 Planned clinical trials of iPS cell-based therapies

Principal investigator (Institute/Location) Cell type to transplant

Masayo Takahashi, (RIKEN) Retinal Pigment Epithel

Alfred Lane, Anthony Oro, Marius Wernig(Stanford University)

Keratinocytes

Mahendra Rao (NIH) DA neurons

Koji Eto (Kyoto University) Megakaryocyte

Jun Takahashi (Kyoto University) DA neurons

Steve Goldman, (University of Rochester) Oligodendrocyte precu

Hideyuki Okano, Masaya Nakamura (Keio University) Neural stem/progenitor

Shigeto Shimmura (Keio University) Corneal endothelial cel

Koji Nishida (Osaka University) Corneal epithelial cells

Yoshiki Sawa (Osaka University) Cardiomyocytes (sheet)

Keiichi Fukuda (Keio University) Cardiomyocytes (sphere

Yoshiki Sasai and Masayo Takahashi (RIKEN) Neuroretinal sheet inclucells

Advanced Cell Technology Megakaryocytes

Representative studies of iPS-based cell therapy with planned clinical trials are listeReferences: [17,19-29].

patient’s HLA type [18]. The development of regenera-tive medicine using iPS cells is being pursued in Japanand the USA for the treatment of patients with retinaldiseases, including age-related macular degeneration [19],spinal cord injuries [17], Parkinson’s disease (PD) [20,21],corneal diseases [22-24], myocardial infarction [25,26], dis-eases that cause thrombocytopenia, including aplasticanemia and leukemia [27,28], as well as diseases such asmultiple sclerosis (MS) and recessive dystrophic epider-molysis bullosa [29] (Table 1).

Regenerative medicine research to discover a treatmentfor spinal cord injury (SCI) by means of iPS celltechnologiesIn 1998, Hideyuki Okano, in collaboration with StevenGoldman, demonstrated for the first time the presenceof neural stem/progenitor cells (NS/PCs) in the adulthuman brain using a neural stem cell marker, the ribo-nucleic acid (RNA)-binding protein Musashi1 [30,31].Research on nerve regeneration then commenced inearnest. That same year, we began regenerative medicineresearch on neural stem cell transplantation in a ratmodel of SCI, and have since made progress in develop-ing NS/PC transplantation therapies in experiments onanimal models of SCI. First, motor function was restoredby transplanting rat fetal central nervous system (CNS)-derived NS/PCs into a rat SCI model [32]. The samestudy also showed that the sub-acute phase is the opti-mal time for NS/PC transplantation after SCI. In thisstudy, at least part of the putative mechanism by whichNS/PC transplantation restored function was identifiedin animal models of SCI. Both the cell autonomous

Target disorders

ium (sheet) Age-related macular degeneration (wet type)

Recessive dystrophic epidermolysis bullosa(RDEB)

Parkinson’s disease

Thrombocytopenia

Parkinson’s disease

rsor cell Multiple Sclerosis

cells Spinal Cord Injury

ls Corneal endothelial dysfunction

(sheet) Corneal epithelial dysfunction and trauma(e.g. Stevens–Johnson syndrome)

Heart Failure

) Heart Failure

ding photoreceptor Retinitis pigmentosa

Refractory thrombocytopenia

d.


effect (such as synaptogenesis between graft-derivedneurons and host-derived neurons) and non-cell autono-mous (trophic) effects mediated cytokines released fromthe graft-derived cells are likely contributing to tissue re-pair and functional recovery. Subsequently, a non-humanprimate SCI model was developed using the commonmarmoset, and motor function in that model was restoredby transplanting human fetal CNS-derived stem cells [33].In the same study, a behavioral assay for motor functionassociated with SCI was developed. Based on these stud-ies, a preclinical research system for cell transplantationtherapy was established in a non-human primate SCImodel.Given these findings, we began preparations for clinical

studies of human fetal CNS-derived NS/PC transplant-ation to treat SCI patients. However, with the guidelinesfor clinical research on human stem cells of the JapaneseMinistry of Health, Labor and Welfare that came intoeffect in 2006, human fetus-derived cells and ES cellsbecame ineligible for use in regenerative medicine. Thus,we had no choice but to change our strategy (human EScells became eligible for use in the 2013 guidelines). In2006, one of our research groups (Yamanaka’s group)established iPS cells from adult mouse skin cells. Hypothe-sizing that it might be possible to induce NS/PCs from iPScells, we (Okano’s group) turned our attention to iPS cellsas a means of obtaining NS/PCs without using fetal or EScells. Based on conditions that were developed for experi-ments on mouse ES cells [34,35], NS/PCs were inducedfrom mouse iPS cells [36]. The following year, we suc-ceeded in restoring motor function by transplanting thesemouse iPS cell-derived NS/PCs into a mouse model ofSCI, and reported that when “good iPS cells” -derived NS/PCs, which had been pre-evaluated as non-tumorigenic bythe transplantation into the brains of immunocomprom-ised mice, were used for transplantation, motor functionwas restored for a long period of time without tumors de-veloping [37]. In 2011, we succeeded in restoring motorfunction by transplanting human iPS cell-derived stemcells into a mouse SCI model [38]. Moreover, in 2012,motor function was restored by transplanting human iPS(line 201B7) cell-derived NS/PCs into the marmosetSCI model, and long-term motor function was recov-ered without observable tumor formation [39]. Thisfinding was of great significance in terms of preclinicalresearch, and provided a proof of concept that couldpotentially lead to a treatment method.Collectively, when mouse or human iPS cells were in-

duced to form NS/PCs and were transplanted into mouseor non-human primate SCI models, long-term restorationof motor function was induced, without tumorigenicity, byselecting a suitable iPS cell line [17,40]. Considering thesub-acute phase (2-4 weeks after the injury) as the optimaltime for iPS cells-derived NS/PCs transplantation for SCI

patients, there are following major difficulties withautograft-based cell therapy. First, it takes about a fewmonths to establish iPS cells. Second, it also takes threemonths to induce them into NS/PCs in vitro. Third,one more year would be required for the quality controlincluding their tumorigenesis.Considering these, our collaborative team (Okano and

Yamanaka laboratories) are currently planning iPS-basedcell therapy for SCI patients in the sub-acute phase usingclinical-grade integration-free human iPS cell lines thatwill be generated by Kyoto University’s Center for iPS CellResearch and Application (CiRA). We will establish a pro-duction method, as well as a storage and management sys-tem, for human iPS cell-derived NS/PCs for use in clinicalresearch for spinal cord regeneration, build an iPS cell-derived NS/PC stock for regenerative medicine, establishsafety screenings against post-transplantation neoplastictransformation, and commence clinical research (PhaseI–IIa) trials for the treatment of sub-acute phase SCI(Figure 3). As these studies progress, the application ofiPS cells to treat chronic phase SCI and stroke will beinvestigated. Significant therapeutic efficacy in the treat-ment of chronic phase SCI has not been achieved by celltransplantation alone [41]. However, clinical studies areplanned using antagonists of axon growth inhibitors,such as Semaphorin3A inhibitors [42], followed bymultidisciplinary rehabilitation combination therapies.We aim to perform a clinical trial based on the Pharma-ceutical Affairs Act in collaboration with drug companiesand to use iPS cell-derived NS/PC stocks for regenerativemedicine to establish treatment methods for stroke, MS,and Huntington’s disease.

iPS cell technologies in nervous system diseaseresearchGeneral statement of human disease modeling with iPScell technologiesLesion sites are difficult to access in patients with degen-erative diseases of the nervous system. Therefore, in paststudies, cell biological or biochemical analyses of theirpathology centered on forced expression of the causativegenes in non-nervous system cultured cell lines and onmice in which the causative gene was knocked out. How-ever, in a few instances, the animal or cell models did notnecessarily reflect the human pathology. Identifying cellbiological or biochemical changes in the initial stages ofthe disease, before onset of symptoms, has been difficultgiven analyses conducted on postmortem brains. However,with the development of iPS cell technologies, it becamepossible to establish pluripotent stem cells from the som-atic cells of anyone, irrespective of race, genetic back-ground, or whether the person exhibits disease symptoms.Thus, it is no exaggeration to say that generation ofdisease-specific iPS cells using iPS cell technologies is the

iPS cells

OCT3/4, SOX2, KLF4,LIN28, p53 DN

Preparation of clinical grade iPS-cell-derived NS/PCs stocks(Okano Group in Keio University)

Neural differentiation and expansion of iPS-derived NS/PCs at the GMP level.Quality control of clinical grade iPS-cell-derived NS/PCs.Freeze and Stocks of clinical grade iPS-cell-derived NS/PCs.

L-MYC,

Figure 3 Strategies for the development of iPS cell-based cell therapy for SCI patients. Our collaborative team (Okano’s group at KeioUniversity and Yamanaka’s group at Kyoto University) have been developing an iPS cell-based cell therapy for SCI since 2006. Our previouspreclinical studies have shown that long-term functional restoration can be obtained by transplantation of NS/PCs derived from appropriate iPScells clones without observable tumor formation [10]. Currently, we aim to develop iPS cells-based cell therapy for SCI patients at sub-acute phaseusing the clinical grade iPS cell-derived NS/PCs (i.e., the role of Okano’s group described in the blue box) which have been prepared from humaniPS cell stock (i.e., the role of Yamanaka’s group described in the yellow box).


sole means of reproducing ex vivo phenomena thatoccur in patients in vivo, particularly for nervous systemdisorders. The result has been a tremendous desire byinvestigators who are conducting research on neurologicaldiseases to become engaged in disease-specific iPS cellresearch [43-45].A variety of disease-specific iPS cells have been used

to study nervous system diseases, including amyotrophiclateral sclerosis (ALS) [46-48], spinal muscular atrophy[49], spinobulbar muscular atrophy [50], Friedreich’s ataxia[51], Alzheimer’s disease (AD) [52-54], PD [55-58],Huntington’s disease [59,60], Machado-Joseph disease[61], fragile X-syndrome [62], Rett’s syndrome [63],familial dysautonomia (FD) [64], Pelizaeus-Merzbacherdisease [65], adrenoleukodystrophy [66], schizophrenia[67-69], and Dravet’s syndrome of intractable epilepsy[70-72] (Table 2). In the following sections, we describethe results of nervous system disease-specific iPS cellresearch using PD and AD as examples [44].

Modeling PD with disease-specific iPS cellsPD is the second most common neurodegenerativedisease after AD. More than 4 million patients areafflicted with PD globally. In Japan, its prevalence isabout 100–150 cases per 0.5 million population [73].PD is characterized by selective degeneration of dopa-minergic (DA) neurons (A9 neurons) in the substantianigra, which results in motor symptoms, including

tremor, rigidity, akinesia, and postural instability. Theremarkable loss of neuromelanin-containing DA neuronsin the substantia nigra pars compacta and the appearanceof Lewy bodies (i.e., eosinophilic intracellular protein-aceous inclusions) are characteristic of PD and relateddiseases [74].L-Dopa therapy or deep brain stimulation are current

methods of treating PD, but they are not curative treat-ments. In most patients, the onset of PD occurs whenthe person is in their late 60s to early 70s; therefore,treatment of PD has become a major task in countriesfacing an aging society. Based on previous research, mu-tation of a specific gene is the cause of PD (familial PD[FPD]) in approximately 10% of PD patients, whereasthe other 90% of patients have sporadic PD [73].Interplay between genetic and environmental factors is

likely to play an important role in the pathogenesis ofPD. In human molecular genetic studies of rare mono-genic forms of PD (FPD), at least 18 loci and 11 genesleading to the development of FPD have been identified.FPD-associated loci and genes for which there is conclu-sive evidence of a role in the disease mechanism includePARK1/PARK4 (α-synuclein [SNCA]), PARK2 (Parkin),PARK6 (PTEN-induced kinase 1 [PINK-1]), PRAK7 (DJ-1),PARK8 (leucine-rich repeat kinase 2 [LRRK2]), and PARK9(ATPase type 13A [ATP13A2]). The gene products areclosely associated with the regulation of mitochondrialfunction and oxidative stress. Environmental risk factors

Table 2 Representative reports on neurological/psychiatric disorders

Name of disease Gene responsible Cells responsible for pathogenesis References

Neurodevelopmental disorders

Rett syndrome MeCP2, CDK5L5 Neurons, neural precursors [63]

Spinomuscular atrophy SMN1 Motor neurons [49]

Familial dysautonomia IKBKAP Neural crest precursor cells [64]

Fragile X-syndrome FMR1 Neurons [62]

Adrenoleuko- dystrophy ABCD1 Oligodendrocytes [66]

Pelizaeus- Merzbacher disease PLP1 Oligodendrocytes [65]

Dravet syndrome SCN1A Neurons [70-72]

Late-onset neurodegenerative disorders

Alzheimer’s disease PS1, PS2, APP, sporadic Neurons [52,53,58,100]

Parkinson’s disease α-synuclein, PARKIN, PINK-1, LRRK2 etc. sporadic Dopaminergic neurons [55-59,83,87]

Amyotrophic lateral sclerosis SOD1, TDP43, FUS, C9ORF etc. sporadic Motor neurons, astroglia [47-49]

Spinobulbar muscular atrophy Androgen receptor Motor neurons, skeletal muscles [50]

Huntington’s disease HTT Glutamatergic neurons, GABAergic neurons [59,60]

Machado-Joseph disease ATX3 Glutamatergic neurons [61]

Psychiatric disorders

Schizophrenia 22q11.2, sporadic Glutamatergic neurons, GABAergic neurons,dopaminergic neurons, etc.

[67-69]

Extracted and modified from References [43,44] and [73].


for PD include drinking well-water, and exposure topesticides, herbicides, and heavy metals (Fe, Cu, andZn). Furthermore, parkinsonism can be experimentallyinduced by administration of certain drugs, includingMPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine),which is metabolized to MPP+ in glial cells and blocksmitochondrial functions. The mechanisms common to theinvolvement of both genetic factors and environmentalfactors in causing PD are mitochondrial dysfunctions andincreased oxidative stress [73]. Notably, there are closeassociations between mitochondrial homeostatic mech-anisms and the gene products encoded by the geneticloci that are correlated with FPD. DA neurons are con-stitutively exposed to oxidative stress, which damagesmitochondria and impairs membrane depolarization.The lowered membrane potential of mitochondria leadsto stabilization of the protein kinase PINK1 (PARK6)[75]. The stabilized PINK-1 then phosphorylates thePARKIN protein (PARK2) (a member of the E3 ubiquitinligase family) [76-78] and recruits it from the cytoplasm tothe outer membrane of the mitochondria, where it ubiqui-tinates mitochondrial outer membrane proteins, includingVDAC1 [79]. Mitochondrial outer membrane proteinsthus tagged are recognized by isolation membranes, whichthen fuse with lysosomes and are ultimately degraded bylysosomal enzymes. In addition, ATP13A2 (PARK9) [80] isan H+-ATPase involved in lysosomal acidification, whichis necessary for lysosome function. Mitochondria damagedby exposure to oxidative stress are degraded by mitophagy.

The mutations in FPD result in impaired mitochondrialhomeostatic mechanisms. We have generated iPS cellsfrom FPD patients [58] to clarify the interaction betweenimpaired mitochondrial homeostatic mechanisms and thedevelopment of PD. Okano’s group has established iPScells from cutaneous fibroblasts obtained from patientswith the PARK2 form of FPD (Patient A: female with anexon 2–4 deletion mutation; Patient B: male with an exon6–7 deletion mutation) by performing retroviral genetransduction (Oct4, Sox2, Klf4, and c-Myc). After selectingseven clones with good neuronal differentiation (PA1,PA9, PA22, PB1, PB2, PB17, and PB20), NS/PCs and tyro-sine hydroxylase (TH)-positive DA neurons were inducedfrom PARK2 patient-derived iPS cells and, as a control,from human ES cells and healthy adult-derived iPScells. In investigating PARK2, patient-derived fibro-blasts, iPS cells, NS/PCs, neurons, and DA neuronswere used to examine multiple aspects, including thetranscriptome, metabolome, proteome, and mitochondrialhomeostasis [58,73].Increased oxidative stress is usually involved in the

pathogenesis of PD; therefore, oxidative stress wasmeasured in PARK2 iPS cell-derived neurons. Thesecells exhibited increased oxidative stress accompaniedby activation of the Nrf2 pathway, which exerts a cytopro-tective role under conditions of reactive oxygen species ac-cumulation. A metabolome analysis of glycolytic pathways,the tricarboxylic acid cycle, and pentose phosphate path-ways suggested that mitochondrial were dysfunctional in


the PARK2-derived neurons (H.O., unpublished results).Further characterization of mitochondria in the PARK2iPS cell-derived neurons revealed abnormal mitochondrialmorphology and impaired mitochondrial turnover.Lewy bodies, a pathology characteristic of PD, and

their main component, α-synuclein, were investigated inPARK2 patient-derived iPS cells [58]. Based on an ana-lysis of patient brain autopsies, Lewy bodies were foundto accumulate in the neurons of patients with sporadicPD. However, while there have been several autopsy re-ports for brains of PARK2 patients, α-synuclein was notgenerally thought to accumulate in the brain in PARK2FPD. When the postmortem brain of a PARK2 patientwas examined histologically, Lewy bodies and aggregatesof α-synuclein were confirmed, and examination ofneuronal cells derived from iPS cells of the same patientrevealed that α-synuclein had accumulated in a similarmanner [58]. These results were the first to demonstratethat patient iPS cell-derived neuronal cells faithfullyreproduced a phenomenon that occurred in the brain ofthe same patient.There is growing interest in genome editing of human

iPS cells in introducing mutations into isogenic iPS cells(reviewed in [81]) and in rescue experiments via geneticrepair, as methods to demonstrate genotype-phenotypecausal relationships in human genetic disorders (discussedin iPS cell technologies in nervous system disease researchand Conclusion) (Figure 2). Various PD-associated ab-normalities, including neurite outgrowth abnormalities,DA neuron death induced by the addition of 6-hydroxydopamine, tau and α-synuclein deposition, andgene expression changes in DA neurons, have been ob-served in DA neurons derived from iPS cells preparedfrom a patient with a LRRK2 gene mutation (G2019Smutation) and in isogenic control iPS cells in which theG2019S mutation was introduced. These PD-associatedphenotypes were rescued by genetic correction of theLRRK2 mutation in the patient-derived iPS cells [82].Studies of PD using iPS cell technology have shown the

presence of PD-associated abnormalities in: 1) mitochon-drial function, 2) the unfolded protein response and Golgito endoplasmic reticulum transport and 3) axonal trans-port and cytoskeletal and neurite extension/retractionresponses (Courtesy of Dr. Ole Isacson, Harvard MedicalSchool). Thus, iPS-cell-based disease modeling is expectedto contribute to the elucidation of the pathophysiology ofPD, and be useful in drug screening and the developmentof methods for extremely early diagnosis, before theappearance of motor symptoms.

ADAD accounts for approximately half of all cases of de-mentia and is the most common intractable neurologicaldisease. AD is usually first manifested by a memory

disorder in a person aged 65 years or older. AD alwaysprogresses to disorientation and a decreased ability tocomprehend and make judgments, and ultimately leadsto personality disorders and a bedfast state. In recentyears, early-onset AD, in which onset occurs from thefifth decade of life onward, has also attracted attention.People with early-onset AD lose the ability to conducttheir daily lives and need long-term care; therefore, thisdisease has become a major social problem. Currenttreatment is primarily symptomatic, and the developmentof curative treatments has been slow. There are currentlyno prospects for a complete cure [44].Based on previous research, it is clear that large

amounts of amyloid beta (Aβ) accumulate in the brainsof AD patients and cause pathological changes called se-nile plaques. Moreover, experiments conducted on cellcultures and mice suggest that the highly toxic Aβ-42may be overproduced in AD. The “amyloid hypothesis”,which states that Aβ is the cause of the disease, has beendifficult to verify in living nerve cells of patients usingprevious technologies. In collaboration with the KeioUniversity neurology department team (Drs. Takuya Yagiand Daisuke Ito, Professor Norihiro Suzuki, and col-leagues), we produced iPS cells from skin fibroblasts offamilial AD (FAD) patients (presenilin (PS)-1 or -2 muta-tions) and succeeded in inducing neuronal cells for thefirst time [52]. We confirmed that these patient-derivedneuronal cells produce twice the normal level of thehighly toxic Aβ-42. This result correlated with Aβ accu-mulation in neural cells derived from living patientswith AD. In addition, we treated AD iPS cell-derivedneuronal cells with a γ-secretase modulator, which is acandidate drug for the treatment of AD, and showedthat production of Aβ-42 was inhibited. Thus, thesedisease-specific iPS cells enabled a novel drug to bedeveloped for the treatment of dementia [44].In a study by Israel et al., Aβ accumulation in neural

cells induced from sporadic AD patient-derived iPS cells[53] was similar to our finding showing that in neuralcells induced from FAD patient-derived iPS cells [52].Accumulation of phosphorylated tau, in addition to Aβ,was observed in the neurons induced from iPS cells ofone of the sporadic AD patients [53]. Gene analysis datawere not described in this report [53], but the phosphor-ylated tau phenotype was not detected in FAD-derivedneurons that had a PS1 or PS2 gene mutation [52].These observations are extremely interesting from thestandpoint of the diversity of AD phenotypes. Based onthese results, AD pathology can be detected in sporadicAD, as well as FAD, and will lead to the development ofnew treatments [44].In addition, Dr. Haruhisa Inoue’s research group at

CiRA produced iPS cells from the skin cells of patientswith a mutation in the amyloid precursor protein (APP)


gene, which is a causative gene in early-onset (familial)AD, and from patients with late-onset (sporadic) ADwho had no family history of AD. The mutant APP iPScells were induced to differentiate into cerebral neurons.When a mutation called APP-E693Δ was present, the Aβprotein formed oligomers, accumulated in cells, inducedendoplasmic reticulum and oxidative stress, and causedinduction of the cell death gene Caspase4. Intracellularstress and neuronal cell death were inhibited by the unsat-urated fatty acid docosahexaenoic acid (DHA), which ispresent at high levels in fish oils. Moreover, intracellularAβ oligomers and cell stress have also been observed insome patients with late-onset sporadic AD, similar tothe APP-E693Δ cells. Based on the analysis of neuronsinduced from iPS cells derived from several patients,there is an AD population in which DHA is effective andan AD population in which is it not. These findings sug-gest a diversity of pathologies in AD and, correspondingly,the need for a diversity of treatment strategies [44,54].Monogenic FAD is rare in comparison to the sporadic

form. However, genetic predisposing factors, for exampleApoE, are present in sporadic AD. ApoE4 is the majorknown genetic risk factor for AD. ApoE is one of the fivemain types of blood lipoproteins (A-E) with 299 aminoacids, with three different isoforms (ApoE2 (Cys112,Cys158), ApoE3 (Cys112, Arg158) and ApoE4 (Arg112,Arg158)) [83]. Genetic polymorphism of the ApoE geneis responsible for the generation of these isoforms [84].While ApoE4 allele is found in approximately 14% of thepopulation [85], the ApoE4 allele is genetically associatedwith late-onset familial and sporadic forms of AD [86],highlighting the importance of ApoE4 in the pathogen-esis of AD. In the future, the role of ApoE4 should becharacterized by a combination of iPS cell technologiesand genome editing.

Future tasks for iPS cell researchers with regard tomodeling human diseasesAs described above, iPS cell researchers have developednew strategies to study the pathophysiology of human dis-eases and to provide assay systems for drug screening.However, many tasks remain to be accomplished to enableiPS cell technologies to accurately model human diseasesand to develop new therapeutic interventions.

a) Development of an accurate assay system fordisease-associated phenotypes
One of the problems with current iPS celltechnologies is that the somatic cells generated fromundifferentiated iPS cells remain immature for longperiods. As a result, iPS cell technologies have beenmost successful in modeling pediatric or early-onsetdiseases, including FD [64], epilepsy (e.g., Dravetsyndrome [70-72]), and Rett syndrome [63]. A
potential problem with current iPS cell technologiesin modeling late-onset neurodegenerative diseases isthe difficulty in obtaining age-related phenotypes ina relatively short timeframe. Lorenz Studer’s grouprecently succeeded in inducing rapid cell aging bymis-expression of progerin, a truncated form oflamin A that is associated with premature aging[87]. This method made it possible to induce agingin PD patient iPS cell-derived DA neurons thatresulted in disease-related phenotypes, includingsevere dendrite degeneration, progressive loss ofTH-positive cells, and abnormal mitochondria orLewy body precursor-like inclusions, which aredifficult to identify using conventional neuronaldifferentiation methods for iPS cells [87]. Notably,we did not observe any Lewy bodies in DA neuronsinduced from PD patient-derived iPS cells [58], eventhough α-synuclein accumulated in progerin-expressing DA neurons in vitro [87] and Lewybodies were prominent in the brain autopsy of thepatient [58]. Although α-synuclein is their majorcomponent, Lewy bodies contain several othercomponents including neurofilaments and ubiquitin[74,88,89]. These observations indicate that Lewybody formation is a dystrophic age-related event,and, thus, Lewy bodies may not form in DA neuronsinduced from PD patient-derived iPS cells byconventional culture methods. Thus, progerin-induced aging is a versatile method to investigate thefeatures of late-onset age-related diseases by iPScell-based disease modeling, while their applicationto other age-related diseases needs to be verified.

b) Demonstration of causative relationships betweengenotypes and phenotypes by genome editingAs a result of the rapid progress in human genomesequencing since the advent of next-generationsequencers, enormous numbers of disease-relatedmutations and single-nucleotide polymorphisms(SNPs) have been identified. Heterogeneity in thegenetic mutations associated with some diseases iscommon. In most diseases, there is no formal proofof a causal relationship between the geneticmutation and the disease phenotype becauseexperimental genetic studies are not possible inhumans, as opposed to animal models, such asDrosophila and mouse. However, demonstration of acausal relationship between a genetic mutation and adisease phenotype can be verified using genomeediting technologies such as the helper-dependentadenoviral vector [90], zinc-finger nucleases [91],transcription activator-like effector nucleases [92,93],or the CRISPR-Cas9 system [94,95] or its improvedmethod [96]. These technologies can be used toperform rescue experiments with gene corrections,


as well as recapitulation of the disease phenotype byintroducing disease-related mutations into controliPS cells [82]. Thus, while genome editing is apowerful technology for demonstrating genotype-phenotype causal relationships, the current genomeediting techniques can only be applied to monogenicdisorders, and new technologies will need to bedeveloped to investigate polygenic disorders.

c) Application to sporadic and common diseasesAlthough Mendelian inheritance patterns have beenwell documented in several neurodegenerativediseases, including PD, AD, and ALS, the majority ofthe cases of these diseases are sporadic, and thegenetic defects responsible for these cases remain tobe identified [97]. In Mendelian diseases, the effectof genetic variation is extremely large, while theallele frequency is extremely low. Thus, Mendeliandiseases are well suited for iPS cell-based diseasemodeling and genome editing. By contrast, themolecular etiology of most sporadic neurodegenerativediseases remains unknown. In a series of studies of thehuman genetics of sporadic diseases, genome-wideassociation studies (GWAS) with SNPs have beenconducted as a means of identifying susceptibilitygenes for sporadic neurodegenerative diseases.Remarkably, these GWAS studies have succeeded inidentifying disease-related rare variants with a highodds ratio [97]. For example, the glucocerebrosidase(GBA) gene polymorphism was identified as arobust genetic risk factor for PD [98]. It will beimportant to characterize the role of GBA mutationsfrom the standpoint of the molecular etiology ofPD, using iPS cell-based in vitro characterization.Since their effect size is not small, such sporadicdiseases with rare genetic variants are also likely tobe suitable targets for iPS cell-based disease modeling.However, it will be important to establish iPS cellsfrom a sufficient number of patients and tocharacterize a large number of clones to performstatistical analyses. Therefore, it will be essential todevelop large-scale automated systems for theproduction and differentiation of iPS cells.

d) Application to preemptive medicineiPS cell-based disease modeling could play animportant role in the early diagnosis of late-onsetneurodegenerative diseases such as AD and PD.Since the motor symptoms of PD do not developuntil almost 70% of the DA neurons in the substantianigra have been lost, the molecular mechanismsthat predominate during the initial stages of PDremain unknown. However, studies characterizingiPS cells derived from the somatic cells of PDpatients have provided an excellent opportunity andexcellent tools to investigate the course of changes

during PD, from the asymptomatic phases throughto the later stages when the pathology has becomeprominent. Such studies could help to develop anappropriate preemptive neuroprotective treatmentfor PD, including small molecules, gene therapy, orcell therapy, which could be started early in theasymptomatic phase. AD usually has a longprogression of more than 30 years that consists ofan asymptomatic phase of ~20 years, a mildcognitive impairment (MCI) phase of ~10 years,and a dementia phase of unlimited length. Amyloidplaques form and continue to enlarge in theasymptomatic phase, and there is already substantialneuronal loss and brain atrophy in the MCI phase[99]. Thus, if diagnosis were possible in theasymptomatic phase, it would provide a greatadvantage by enabling the use of treatments toprevent dementia, including γ-secretase modulators[52], Non-Steroidal Anti-Inflammatory Drugs(NSAIDs) [100], and DHA [54]. A combination ofiPS cell-based phenotypic screening, whole genomesequencing by next-generation sequencing toidentify AD-related polymorphisms, and imaging ofAβ and tau by positron emission tomography wouldenable reliable diagnosis of AD in the asymptomaticphase. While obtaining a proof of concept for suchpreemptive treatments of AD would be difficult toobtain in a short time, we hope that such data canbe obtained by using a combination of large-scaleiPS cell-based disease modeling and a cohort studyof dominantly inherited FAD, similar to theDominantly Inherited Alzheimer Network (DIAN)study [101-103]. The development of preemptivetreatments for late-onset neurodegenerativediseases would be enormously important in rapidlyaging countries like Japan.

ConclusionsAs described above, since 2006, there have been enormousprogresses in iPS cell technologies aiming for medical sci-ence, in both regenerative medicine and human diseasemodeling. Furthermore, iPS cell technologies could be ap-plied for preemptive medicine. However, it is also true thatiPS cell technologies have not yet saved any patients’ livesat this moment in early 2014. Continuous efforts throughthe cooperation of basic stem cell biology, clinical investi-gation of diseases, translational research, pharmaceuticalscience, regulatory science and system biology will be ne-cessary to let iPS cells really contribute to human health.

Competing interestsH.O. is a paid scientific consultant to San Bio, Inc., Eisai Co., Ltd., and DaiichiSankyo Co., Ltd. S.Y. is a member without salary of the scientific advisoryboards of iPierian, iPS Academia Japan, Megakaryon Corporation andHEALIOS K.K. Japan.


Authors’ contributionsBoth HO and SY wrote the manuscript and conducted the researchesrelevant to the present paper. All authors read and approved the finalmanuscript.

AcknowledgementsWe thank all of the members of the Okano and Yamanaka laboratories fortheir encouragement and support. We are also grateful to Dr. MasayaNakamura for providing a prototype graphic for Figure 3 and Prof. OleIsacson at Harvard Medical School for valuable discussions. This study wassupported by grants from the Program for Intractable Disease ResearchUtilizing Disease-specific iPS Cells funded by the Japan Science andTechnology Agency (JST) to H.O. and S.Y.

Author details1Department of Physiology, Keio University School of Medicine, 35Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan. 2Center for iPS CellResearch & Application, Kyoto University, 53 Kawaharacho, Shogoin, Sakyoku,Kyoto 606-8507, Japan. 3Gladstone Institute of Cardiovascular Disease, SanFrancisco, CA 94158, USA.

Received: 14 February 2014 Accepted: 26 March 2014Published: 31 March 2014

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doi:10.1186/1756-6606-7-22Cite this article as: Okano and Yamanaka: iPS cell technologies:significance and applications to CNS regeneration and disease.Molecular Brain 2014 7:22.

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